Sensing absolute position of an encoded object

ABSTRACT

In one aspect of the present invention, a device for sensing an absolute position of an encoded object, comprising: a position tracking module comprising: a track illumination module configured to illuminate the encoded object with one more light beams, and to detect one or more light beams resulting from said illumination of said encoded object; and an absolute position determinator configured to determine the absolute position of the encoded object based on said one or more light beams resulting from said illumination of said encoded object.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application makes reference to and claims the benefit of thefollowing co-pending U.S. Provisional Patent Application No. 60/684,531filed May 26, 2005. The entire disclosure and contents of the foregoingProvisional Application is hereby incorporated by reference. Thisapplication also makes reference to the following co-pending U.S. PatentApplications. The first application is U.S. application Ser. No.11/440,370, entitled “Illuminative Treatment of Holographic Media,”filed May 25, 2006. The second application is U.S. application Ser. No.11/440,446, entitled “Methods and Systems for Laser Mode Stabilization,”filed May 25, 2006. The third application is U.S. application Ser. No.11/440,447, entitled “Phase Conjugate Reconstruction of Hologram,” filedMay 25, 2006. The fourth application is U.S. application Ser. No.11/440,448, entitled “Improved Operational Mode Performance of aHolographic Memory System,” filed May 25, 2006. The fifth application isU.S. application Ser. No. 11/440,359, entitled “Holographic Drive Headand Component Alignment,” filed May 25, 2006. The sixth application isU.S. application Ser. No. 11/440,358, entitled “Optical Delay Line inHolographic Drive,” filed May 25, 2006. The seventh application is U.S.application Ser. No. 11/440,357, entitled “Controlling the TransmissionAmplitude Profile of a Coherent Light Beam in a Holographic MemorySystem,” filed May 25, 2006. The eighth application is U.S. applicationSer. No. 11/440,371, entitled “Sensing Potential Problems in aHolographic Memory System,” filed May 25, 2006. The ninth application isU.S. application Ser. No. 11/440,367, entitled “Post-Curing ofHolographic Media,” filed May 25, 2006. The tenth application is U.S.application Ser. No. 11/440,366, entitled “Erasing Holographic Media,”filed May 25, 2006. The eleventh application is U.S. application Ser.No. 11/440,365, entitled “Laser Mode Stabilization Using an Etalon,”filed May 25, 2006. The twelfth application is U.S. application. Ser.No. 11/440,369, entitled “Holographic Drive Head Alignments,” filed May25, 2006. The thirteenth application is U.S. application Ser. No.11/440,368, entitled “Replacement and Alignment of Laser,” filed May 25,2006. The entire disclosure and contents of the foregoing U.S. PatentApplications are hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention relates generally to sensing the position of anobject, and more particularly, to sensing absolute position of anencoded object.

2. Related Art

Developers of information storage devices continue to seek increasedstorage capacity. As part of this development, memory systems employingholographic optical techniques, referred to herein as holographic memorysystems, have been suggested as alternatives to conventional memorydevices.

Typically, holographic memory systems read/write data to/from aphotosensitive storage medium. Such systems typically access holographicrepresentations (i.e., holograms) substantially throughout the spatialextent of the storage medium. This allows holographic memory systems toadvantageously store a large amount of data.

Holographic memory systems may be designed to record data as single bitsof information (i.e., bit-wise data storage). See McLeod et al.“Micro-Holographic Multi-Layer Optical Disk Data Storage,” InternationalSymposium on Optical Memory and Optical Data Storage (July 2005).Holographic memory systems may also be designed to record an array ofdata that may be a 1-dimensional linear array (i.e., a 1×N array, whereN is the number linear data bits), or a 2-dimension array commonlyreferred to as a “page-wise” memory system. Page-wise memory systems mayinvolve the storage and readout of an entire two-dimensionalrepresentation (i.e., a page) of data.

Holographic memory systems typically involve the three-dimensionalstorage of holograms as a pattern of varying refractive index and/orabsorption imprinted into the storage medium. In general, holographicmemory systems operate to perform a data write (also referred to as adata record or data store operation, simply “write” operation herein) bycombining two coherent light beams at a particular point within thestorage medium. Specifically, a data-encoded light beam is combined witha reference light beam to create an interference pattern in thephotosensitive storage medium. The interference pattern induces materialalterations in the storage medium to form a hologram. The formation ofthe hologram is a function of the relative amplitudes, phase, coherence,and polarization states of the data-encoded and reference light beams.It is also dependent on the relative wavelength of the incident beams aswell as the three-dimensional geometry at which the data and referencebeams are projected into the storage medium.

Holographically-stored data is retrieved from the holographic memorysystem by performing a read (or reconstruction) of the stored data. Theread operation is performed by projecting a reconstruction or probe beaminto the storage medium at the same angle, wavelength, phase andposition as the reference beam used to record the data, or compensatedequivalents thereof. The hologram and the reconstruction beam interactto reconstruct the data beam. The reconstructed data beam is thendetected by a sensor, such as a photo-detector, sensor array, camera,etc. The reconstructed data is then processed for delivery to an outputdevice.

In order to achieve proper operation of a holographic memory system, theholographic memory system must determine the position of the holographicstorage medium relative to the optical components of the system. Thus,it is desirable to quickly and accurately determine the position of theholographic storage medium.

SUMMARY

In one aspect of the present invention, a device for sensing an absoluteposition of an encoded object, comprising: a position tracking modulecomprising: a track illumination module configured to illuminate theencoded object with one more light beams, and to detect one or morelight beams resulting from said illumination of said encoded object; andan absolute position determinator configured to determine the absoluteposition of the encoded object based on said one or more light beamsresulting from said illumination of said encoded object.

In another aspect of the present invention, a holographic memory systemis disclosed. The storage system comprises an encoded recording mediumconfigured to holographically store information; at least one source ofcoherent light; and a position tracking module comprising: a trackillumination module configured to illuminate said encoded storage mediumwith one more light beams, wherein said track illumination module isconfigured to detect one or more light beams resulting from saidillumination of said encoded storage medium; and an absolute positiondeterminator configured to determine the absolute position of saidencoded storage medium based on said one or more light beams resultingfrom said illumination of said encoded storage medium.

In a further aspect of the present invention, a method of determiningthe absolute position of an encoded medium comprising: illuminating saidencoded medium with one or more light beams; detecting the resultingillumination from said encoded medium; and determining the absoluteposition of said encoded medium based on the detected illumination fromsaid encoded medium.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in conjunctionwith the accompanying drawings, in which:

FIG. 1 is schematic block diagram of an exemplary holographic memorysystem in which embodiments of the present invention may beadvantageously implemented;

FIG. 2A is an architectural block diagram of the components of aholographic memory system illustrating the optical paths utilized duringa write operational mode of the holographic memory system in accordancewith one embodiment of the present invention;

FIG. 2B is an architectural block diagram of the components of aholographic memory system illustrating the optical paths utilized duringa read operational mode of the holographic memory system in accordancewith one embodiment of the present invention;

FIG. 3A is a functional block diagram of components of a positiontracking module as may be implemented in a holographic memory systemsuch as that illustrated in FIGS. 1, 2A, and 2B, in accordance with oneembodiment of the present invention;

FIG. 3B is a functional block diagram of components of a positiontracking module as may be implemented in a holographic memory systemsuch as that illustrated in FIGS. 1, 2A, and 2B, in accordance withanother embodiment of the present invention;

FIG. 4 is a high level flowchart in accordance with one embodiment ofthe present invention;

FIG. 5 is an enlarged schematic diagram illustrating a small segment ofan encoded holographic storage disk in accordance with embodiments ofthe present invention;

FIG. 6 is an architectural block diagram of an LFSR used to generate thepattern used to encode portions of a holographic storage disk inaccordance with embodiments of the present invention;

FIG. 7 is an architectural block diagram of the components of aholographic memory system illustrating the location of a positiontracking module in accordance with one embodiment of the presentinvention;

FIG. 8 is an enlarged functional block diagram of the components of aposition tracking module in accordance with the embodiments of thepresent invention illustrated in FIG. 3A;

FIG. 9 is an enlarged functional block diagram of the components of aposition tracking module in accordance with the embodiments of thepresent invention illustrated in FIG. 3B;

FIG. 10 is an enlarged block diagram illustrating a detector used inembodiments of the present invention; and

FIG. 11 is an enlarged block diagram illustrating the diffractionpattern observed by the detector illustrated in FIG. 10 in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION

Aspects of the present invention are generally directed to a positiontracking module configured to sense the absolute position of an encodedobject. The position tracking module comprises a track illuminationmodule and an absolute position determinator. The track illuminationmodule comprises a light source that directs a light beam towards one ormore tracks on the object, and a light steering subsystem positionedbetween the light source and the object to form the source light beaminto one or more light beams, and to direct each such light beam to oneof the track(s) on the object. The track illumination module alsocomprises a detector module. In certain embodiments, the detector moduledetects a diffraction pattern resulting from a light beam reflected froma track on the object. In other embodiments, the detector module detectsa diffraction pattern resulting from a light beam transmitted by a trackon the object. The detector module provides the absolute positiondeterminator with a signal resulting from the detected diffraction.Absolute position determinator determines the absolute position of themedium based on a diffraction pattern signal from the detector module.

In certain applications, the present invention is embodied in a positiontracking module configured to sense the absolute position of aholographic storage medium in a holographic memory system; that is, adata storage and retrieval system that implements holographic opticaltechniques.

FIG. 1 is a block diagram of an exemplary holographic memory system inwhich embodiments of the present invention may be implemented. It shouldbe appreciated that although embodiments of the present invention willbe described in the context of the exemplary holographic memory systemshown in FIG. 1, the present invention may be implemented in connectionwith any system now or later developed that implement holographicoptical techniques.

Holographic memory system 100 receives along signal line 118 signalstransmitted by an external processor 120 to read and write data to aphotosensitive holographic storage medium 106. As shown in FIG. 1,processor 120 communicates with drive electronics 108 of holographicmemory system 100. Processor 120 transmits signals based on the desiredmode of operation of holographic system 100. For ease of description,the present invention will be described with reference to read and writeoperations of a holographic memory system. It should be apparent to oneof ordinary skill in the art, however, that the present inventionapplies to other operational modes of a holographic memory system, suchas Pre-Cure, Post-Cure, Write Verify, or any other operational modeimplemented now or in the future in a holographic memory system.

Using control and data information from processor 120, drive electronicsmodule 108 transmits signals along signal lines 116 to variouscomponents of holographic memory system 100. One such component thatreceives signals from drive electronics 108 is coherent light source102. Coherent light source 102 may be any light source now or laterdeveloped that generates a coherent light beam. In one embodiment of theinvention, coherent light source 102 is a laser.

The coherent light beam from light source 102 is directed along lightpath 112 into an optical steering subsystem 104. Optical steeringsubsystem 104 directs one or more coherent light beams along one or morelight paths 114 to holographic storage medium 106. In the writeoperational mode described further below, at least two coherent lightbeams are transmitted along two light paths 114 to create aninterference pattern in holographic storage medium 106. The interferencepattern induces material alterations in storage medium 106 to form ahologram, as described in further detail below.

In the read operational mode, holographically-stored data is retrievedfrom holographic storage medium 106 by projecting a reconstruction orprobe beam along at least one light path 114 into storage medium 106.The hologram and the reconstruction beam interact to reconstruct thedata beam which is transmitted along light path 122. The reconstructeddata beam is detected by a sensor 110. Sensor 110 may be any type ofdetector known or used in the art suitable for detecting a coherentlight beam, such as a camera, photodetector, and the like.

The light detected at sensor 110 is converted to a signal andtransmitted to drive electronics 108 via data line 124. Processor 120then receives the requested data and/or related information from driveelectronics 108 via signal line 118.

A more detailed description of the components of an exemplary embodimentof holographic memory system 100 is presented next below with referenceto FIGS. 2A and 2B. This embodiment of holographic memory system 100 isreferred to herein as holographic memory system 200. FIGS. 2A and 2B aresimilar schematic block diagrams of the components of one embodiment ofholographic memory system 200 illustrating the optical paths utilizedduring write and read operations, respectively.

It should be appreciated by those of ordinary skill in the art that theembodiment of optical steering subsystem 104 depicted in FIGS. 2A and 2Bis exemplary only and that the holographic memory system in whichembodiments of the present invention may be implemented may compriseother components to holographically store data in a photosensitivestorage medium. For example, embodiments of the present invention may beimplemented in an optical steering subsystem 104 that implements ahologram multiplexing scheme without any moving parts.

Referring to the write mode configuration illustrated in FIG. 2A,coherent light source 102 (FIG. 1) is a laser 204. Laser 204 receivesvia signal line 116 control signals from an embodiment of driveelectronics 108 (FIG. 1), referred to as drive electronics 202. In theillustrated write mode configuration, such a control signal causes laser204 to generate a coherent light beam 201 which is directed along lightpath 112, introduced above with reference to FIG. 1.

Coherent light beam 201 is reflected by mirror 290 and is directedthrough optical shutter 276. Optical shutter 276 comprises beamdeviation assembly 272, focusing lens 274 and pinhole 206 that arecollectively controllable to shutter coherent light beam 201 fromentering the remainder of optical steering subsystem 104.

Coherent light beam 201 passing through optical shutter 276 enters mainexpander assembly 212. Main expander 212 includes lenses 203 and 205 toexpand the light beam to a fixed diameter and to spatially filter thelight beam. An exposure shutter 208 within main expander assembly 212 isan electromechanical device that controls recording exposure times.

Upon exiting main expander assembly 212, coherent light beam 201 isdirected through an apodizer 210. As is well-known in the art, lightemitted from a laser such as laser 204 has a generally Gaussiandistribution of light. Apodizer 210 converts the Gaussian beam 201 fromlaser 204 into a more uniform beam with controlled edge profiles.

After passing through apodizer 210, coherent light beam 201 entersvariable optical divider 214. Variable optical divider 214 uses adynamically-controlled polarization device 218 and at least onepolarizing beam splitter (PBS) 216 to redirect coherent light beam 201into one or more discrete light beams transmitted along two light paths114 (FIG. 1): light path 260 and light path 262. Variable opticaldivider 214 dynamically allocates the power of coherent light beam 201among these discrete light beams 280, 282. In the write operational modeshown in FIG. 2A, the discrete light beam directed along light path 260is the noted reference light beam, referred to as reference light beam280 (also referred to herein as reference beam 280), while the discretelight beam directed along light path 262 is the noted data light beam,referred to as data light beam 282 (also referred to herein as data beam282).

Upon exiting variable optical divider 214, reference beam 280 isreflected by mirror 291 and directed through beam-shaping device 254Apositioned in reference path 260. After passing through beam shapingdevice 254A, reference beam 280 is reflected by mirrors 292 and 293towards galvo mirror 252. Galvo mirror 252 reflects reference beam 280into scanner lens assembly 250. Scanner lens assembly 250 has lenses219, 221, 223 and 225 to pivotally direct reference beam 280 atholographic storage media 106, shown as holographic storage disk 238 inFIGS. 2A and 2B.

Returning attention to variable optical divider 214, data light beam 282exits the variable optical divider and passes through data beam expanderlens assembly 220. Data beam expander 220 implements lenses 207 and 209to magnify data beam 282 to a diameter suitable for illuminating SpatialLight Modulator (SLM) 226, located further along data beam path 262.Data beam 282 then passes through phasemask 222 to improve theuniformity of the Fourier transform intensity distribution. Data beam282 is then imaged to SLM 226 via 1:1 relay 224 having lenses 211 and213, and PBS 258.

SLM 226 modulates data beam 282 to encode information into the databeam. SLM 226 receives the encoding information from drive electronics202 via a signal line 116. Modulated data beam 282 is reflected from SLM226 and passes through PBS 258 to a switchable half-wave plate 230.Switchable half-wave plate 230 is used to optionally rotate thepolarization of data beam 282 by approximately 90 degrees. A 1:1 relay232 containing beam-shaping device 254B and lenses 215 and 217 directsdata beam 282 to storage lens 236 which produces a filtered Fouriertransform of the SLM data inside holographic storage disk 238.

At a particular point within holographic storage disk 238, referencebeam 280 and data beam 282 create an interference pattern to record ahologram in holographic storage disk 238.

Holographic memory system 100 further comprises an illuminative curingsubsystem 242. Illuminative curing subsystem 242 is configured toprovide a uniform curing light beam with reduced coherence toholographic storage disk 238 to pre-cure and/or post-cure a region ofthe storage medium. Illuminative curing subsystem 242 comprises a laser256 sequentially aligned with a diffuser 244, a lenslet array 243 and alens 229. Laser 256 provides a high intensity illuminative curing lightbeam that is incident on storage disk 238. The light from laser 256 isprocessed by diffuser 244, lenslet array 243, and lens 229 prior toreaching holographic storage disk 238.

Holographic system 100 additionally comprises an associative read afterwrite (ARAW) subsystem 248. ARAW subsystem 248 is configured topartially verify a hologram soon after the hologram is written tostorage medium 106. ARAW subsystem comprises a lens 227 and a detector246. Holographic system 100 uses ARAW subsystem 248 by illuminating awritten hologram with an all-white data page. When a hologram isilluminated by this all-white data page, ARAW subsystem 248 detects thereconstructed reference beam resulting from this all-white illumination.Specifically, detector 246 examines the reconstructed reference beam toverify that the hologram has been recorded correctly.

Referring to the read mode configuration illustrated in FIG. 2B, laser204 generates coherent light beam 201 in response to control signalsreceived from drive electronics 202. As noted above, coherent light 201is reflected by mirror 290 through optical shutter 276 that shutterscoherent light beam 201 from entering the remainder of optical steeringsubsystem 104. Coherent light beam 201 thereafter enters main expanderassembly 212 which expands and spatially filters the light beam, asdescribed above with reference to FIG. 2A. Upon exiting main expanderassembly 212, coherent light 201 is directed through apodizer 210 toconvert the Gaussian beam into a more uniform beam.

In the arrangement of FIG. 2B, when coherent light beam 201 entersvariable optical divider 214, dynamically-controlled polarization device218 and PBS 216 collectively redirect the coherent light beam into onediscrete light beam 114, referred to as reconstruction beam 284.Reconstruction data beam 284 travels along reconstruction beam path 268,which is the same path 260 traveled by reference beam 280 during thewrite mode of operation, described above with reference to FIG. 2A.

A desired portion of the power of coherent light beam 201 is allocatedto this single discrete light beam based on the selected polarizationimplemented in device 218. In certain embodiments, all of the power ofcoherent light beam 201 is allocated to reconstruction light beam 284 tomaximize the speed at which data may be read from holographic medium238.

Upon exiting variable optical divider 214, reconstruction beam 284 isdirected by mirror 291 through beam-shaping device 254A.

After passing through beam-shaping device 254A, reconstruction beam 284is directed to scanner lens 250 by mirrors 292 and 293, and galvo mirror252. Scanner lens assembly 250 pivots reconstruction beam 284 at adesired angle toward holographic storage disk 238.

During the read mode, reconstruction beam 284 passes through holographicstorage disk 238 and is retro-reflected back through the medium by agalvo mirror 240. As shown in FIG. 2B, the data reconstructed on thissecond pass through storage disk 238 is directed along reconstructeddata beam path 298 as reconstructed data beam 264.

Reconstructed data beam 264 passes through storage lens 236 and 1:1relay 232 to PBS 258, all of which are described above with reference toFIG. 2A. PBS 258 reflects reconstructed data beam 264 to an embodimentof sensor 110 (FIG. 1), here a camera 228. The light detected by camera228 is converted to a signal and transmitted to drive electronics 202via signal line 124, introduced above with reference to FIG. 1.Processor 120 then receives the requested data and/or relatedinformation from drive electronics 202 via signal line 118.

Embodiments of the position tracking module of the present inventionwill now be described in detail with reference to an exemplaryholographic memory system illustrated in FIGS. 3A-11. In the embodimentsillustrated in FIGS. 3A-11, an encoded object is referred to as aholographic storage disk 238.

FIGS. 3A and 3B are functional block diagrams of components of aposition tracking module as may be implemented in a holographic memorysystem such as that illustrated in FIGS. 1, 2A and 2B, to determine theabsolute position of holographic storage disk 238. As shown in FIGS. 3Aand 3B, holographic storage disk 238 is positioned to lie in an idealimaginary plane. This is illustrated FIGS. 3A and 3B as a rectangularportion of storage disk 238 lying in a plane 302. Plane 302 is definedby orthogonal X- and Y-axes 306, 304. Orthogonal to plane 302 is aZ-axis 308. When holographic storage disk 238 is positioned inholographic memory system 200, the storage disk 238 rotates within plane302 about Z-axis 308, as shown by arrow 310 to enable the system toperform read, write and other operations with the storage disk. In suchembodiments, holographic storage disk 238 is typically a round disk. Inother embodiments, holographic storage disk 238 would translate alongthe X- or Y-axis 306, 308 during normal operations. In theseembodiments, holographic storage disk 238 is typically a square storagedisk.

As noted, position tracking module 350 is configured to sense theabsolute position of an encoded object such as a holographic storagemedium which, in the context of holographic memory system 200, is aholographic storage disk 238. The term “absolute position” as usedherein refers to the position of holographic storage disk 238 withinplane 302. As a result, embodiments of position tracking module 350detect translation and/or rotation of holographic storage disk 238within plane 302. By determining the position of holographic storagedisk 238 within plane 302, holographic memory system 200 can quickly andaccurately determine where to write to and/or read from holographicstorage disk 238, as well as determine the correct position forperforming various other operations.

Referring to FIG. 3A, position tracking module 350 comprises a trackillumination module 344A and an absolute position determinator 370.Track illumination module 344A comprises a light source 340, a lightsteering subsystem 360A and a detector module 380A. Light source 340generates a light beam 362. In this particular application of aholographic memory system, light beam 362 is at least a partiallycoherent light beam. Light steering subsystem 360A is disposed betweenlight source 340 and holographic storage disk 238, and is configured todirect coherent light beam 362 towards holographic storage disk 238.Light steering subsystem 360A forms coherent light beam 362 into one ormore coherent light beams 366A each of which is transmitted along acorresponding one or more optical path(s) 364.

The one or more light beams 366A traveling along optical path(s) 364 areincident upon holographic storage disk 238, and is/are at leastpartially reflected by holographic storage disk 238 back towards lightsteering subsystem 360A. Light steering subsystem 360A directs thereflected light beam(s) to detector module 380A. In alternativeembodiments, the light beam is reflected from holographic storage disk238 are not reflected back through light steering subsystem 360A.Rather, the light beam is reflected at an angle from holographic storagedisk 238 directly to detector module 380A.

The light pattern detected at detector module 380A is converted to asignal which is relayed via data line 124 to absolute positiondeterminator 370. Absolute position determinator 370 is configured todetect the absolute position of holographic disk 238 based on the lightreflection pattern (described below) at detector module 380. Absoluteposition determinator 370 generates a signal 376 representing theabsolute position of holographic storage disk 238 for use by othercomponents of holographic memory system 200. Details of the aboveelements of track illumination module 344A are provided below.

Referring to FIG. 3B, position tracking module 350 comprises a trackillumination module 344B and an absolute position determinator 370.Track illumination module comprises a coherent light source 340, a lightsteering subsystem 360B and a detector module 380B.

As described above with reference to FIG. 3A, light source 340 generatesa coherent light beam 362. Light steering subsystem 360A is disposedbetween light source 340 and holographic storage disk 238, and isconfigured to direct coherent light beam 362 towards holographic storagedisk 238. Light steering subsystem 360A forms coherent light beam 362into one or more coherent light beams 366A each of which is transmittedalong a corresponding one or more optical path(s) 364.

In the embodiment illustrated in FIG. 3B, the light beam(s) travelingalong optical path 364 are at least partially transmitted by holographicstorage disk 238. This transmitted light beam 378 is then incident ondetector module 380B. The light pattern detected at detector module 380Bis converted to a signal and relayed to absolute position determinator370. Absolute position determinator 370 is configured to detect theabsolute position of holographic disk 238 based on the lighttransmission at detector module 380.

It would be appreciated that light sources other than a laser could beused in embodiments of the present invention. For example, light source340 could be any source that generates a light beam with minimalcoherence suitable for use in the particular embodiment of detectormodule 380. Furthermore, it would be appreciated that it is an advantageof the present invention that a light source having less coherency andpower requirements than the light sources used in the read, write andother operational modes may be used as light source 340 in trackillumination module 344.

Although embodiments of the present invention are described withreference to a track illumination module 344, it would be appreciatedthat the components and function of track illumination module may beimplemented in various other forms including other combinations ofsoftware and/or hardware. In the above embodiment a single light source340 is utilized to generate a source light beam 362 which is then formedby the light steering subsystem 360A into one or more light beams 366A.It should be appreciated, however, that in alternative embodimentsmultiple light sources 340 may be used, each generating a source lightbeam 362 which is formed into one or more of the light beams 366A.

FIG. 4 is a high level flowchart in accordance with one embodiment ofthe present invention. In accordance with embodiments of the presentinvention, at block 402, holographic memory system 200 receives anencoded holographic storage disk 238. Details of encoded holographicstorage disk 238 are provided below with reference to FIGS. 5 and 6.

At block 404, holographic memory system 200 illuminates holographicstorage disk 238 with one or more light beams.

At block 406, the reflected or transmitted light resulting from theillumination of holographic storage disk 238 with the one or more lightbeams is detected at a detector module.

At block 408, based on the reflection or transmission of the one or morelight beams, an absolute position detector determines the absoluteposition of holographic storage disk 238

FIG. 5 is an enlarged schematic diagram illustrating a small segment ofan encoded pattern on a holographic storage disk in accordance with oneembodiment of the present invention. As noted above, the absoluteposition of holographic storage disk 238 is sensed by illuminatingtracks on the disk. In one embodiment, these tracks comprise twoquadrature tracks 510, 530 and one address track 520 each embossed intoholographic storage disk 238. The embossing may consist of any methodthat creates an optical path difference between embossed and unembossedregions such as that introduced by etching. In the embodiment describedherein, the tracks are etched into holographic storage disk 238 usingany etching technique now or later developed. As such, the etchingtechnique is not described further herein.

Referring first to quadrature tracks 510, 530, illumination of thesetracks results in the reconstruction of sinusoidal curves. Illuminationof track 510 results in the reconstruction of a sine curve. As such,track 510 is referred to herein as sine track 510. Illumination of track530 results in the reconstruction of a cosine curve. As such, track 530is referred to herein as cosine track 530.

Tracks 510 and 530 comprise an etched pattern of alternating grooves andmesas. Etched grooves are shown in FIG. 5 as dark rectangles while mesasare shown in FIG. 5 as the areas between these rectangles. Asillustrated, cosine track 530 is etched so as that the reconstructedcosine curve from track 530 is shifted 90 degrees from the reconstructedsine curve from track 510. The reconstruction and functions of thequadrature tracks are described below with reference to FIG. 11.

Address track 520 is a pseudo-random pattern of etched grooves andmesas. Etched grooves are shown dark rectangles while the dottedrectangles indicate a position where a groove is absent. Address track520 is aligned with sine track 510.

As one of ordinary skill will find apparent, the detected change inabsolute position (translation and/or rotation) is based on theconfiguration of tracks 590 on the object. That is, linear tracks areutilized to detect translational changes in absolute position parallelto such tracks. Similarly, circular tracks are utilized to detectrotational changes in absolute position of the object.

The pseudo-random pattern of address track 520 is determined in oneembodiment by a linear feedback shift register (LFSR). FIG. 6 is aschematic diagram of one embodiment of an LFSR 600 used to generate thepattern used to encode portions of holographic storage disk 238.

LFSR 600 is composed of a serial configuration of n D flip-flops 604 andan XOR gate 602. The next state of the LFSR 600 is a function of itscurrent state shifted by 1 bit. As such, two adjacent states generatedby LFSR 600 share n−1 bits, wherein n equals the number of flip-flopsused in the LFSR, and any n bit subsequence of the output sequencerepresents an n bit state of the LFSR. The number of pseudo-randomstates generated by the LFSR equals (2^(n)−1) provided that the XORfunction is selected to effect division by a primitive polynomial. Thus,the LFSR will cycle through all (2^(n)−1) possible non-zero statesexactly once per (2^(n)−1) clock cycles, and every n bit non-zero statewill appear exactly once as a subsequence of the (2^(n)−1) bit outputsequence when it is treated as a circular (wrapping) sequence.Furthermore, the circular (2^(n)−1) bit output sequence will containexactly one subsequence of exactly n−1 zeros in a row. If another zerois inserted into this subsequence, then the resulting 2^(n) bit circularsequence will contain every possible n bit subsequence exactly once,including the n-zeros subsequence. General aspects of LFSRs are known inthe art and will not be described in further detail. In the embodimentshown in FIG. 6, the serial configuration of D flip-flops 604 comprises12 D flip-flops. Thus, in this embodiment, n equals 12 and LFSR 600 willproduce a pseudo-random pattern having (2^(n)−1), or 4095 differentunique n bit subsequences. In another embodiment, a zero is inserted toproduce a pseudo-random pattern having 2^(n), or 4096 different unique nbit subsequences.

Initialization of LFSR 600 is performed by loading the whole LFSR froman initialization register, or possibly by using reset and preset inputsinto the flip-flops. In alternative embodiments, LFSR 600 may beimplemented partially or wholly in software.

FIG. 7 is an architectural block diagram of the components ofholographic memory system 200 illustrating the location of a positiontracking module in accordance with one embodiment of the presentinvention. Position tracking module 750 operates simultaneously withother operational modes of holographic memory system 200 and is thuspositioned so as not to interfere with other drive operations. In theembodiments shown in FIG. 7, the light paths corresponding to otheroperational modes as discussed above have been removed for clarity. Itwould be appreciated by one of ordinary skill in the art that positiontracking module 750 may be place in other locations within holographicmemory system 200.

FIG. 8 is an enlarged functional block diagram of the components of oneembodiment of position tracking module 350, introduced above withreference to FIG. 3A. As noted, position tracking module 350 comprises atrack illumination module 344A and an absolute position detector 370. Inaccordance with the embodiment illustrated in FIG. 8, track illuminationmodule 344A comprises a coherent light source 840, a light steeringsubsystem 360A, and a detector module 380A.

Light source 840 comprises a laser and is referred to as laser 840.Laser 840 generates a coherent light beam 830 directed towardsholographic storage disk 238. Light beam 830 enters light steeringsubsystem 360A positioned between laser 840 and holographic storage disk238. Light steering subsystem 360A comprises a lens 842, a grating 844,a polarizing beam splitter (PBS) 846 and a quarter wave plate 856sequentially aligned along a single light path, here a light pathbetween laser 840 and holographic storage disk 238.

In some embodiments, lens 842 is positioned between laser 840 andgrating 844 in the path of coherent light beam 830. Lens 842 ispositioned such that portions of coherent light beam 830 incident onholographic storage disk 238 will be focused coherent beam spots.

It would be appreciated by one of ordinary skill in the art that lens842 could be positioned in the path of coherent light beam 830 aftergrating 844. Similarly, it would be appreciated by one of ordinary skillin the art that track illumination module 344A may also operate withoutlens 842 positioned in light steering subsystem 360A. In saidembodiment, the portions of coherent light beam 830 incident onholographic storage disk 238 would not be focused beam spots.

Coherent light beam 830 leaving lens 842 then impinges on grating 844.Grating 844 is a diffractive grating that forms or diffracts coherentlight beam 830 into one or more coherent light beams 832 directedtowards holographic storage disk 238. In the illustrated embodiment inwhich a plurality of coherent light beams 832 are desired, grating 844diffracts (i.e., forms or divides) coherent light beam 830 into thatquantity of coherent light beams 832. For example, in the embodimentsherein, grating 844 diffracts coherent light beam 830 into threecoherent light beams 832. Grating 844 also allocates the power of lightbeam 830 between coherent light beams 832, as is well-known in the art.

Coherent light beams 832 are then incident on PBS 846. The polarizationorientation of coherent light beams 832 are such that PBS 846 willsubstantially passes through or transmits the light beams.

Coherent light beams 832 leaving PBS 846 then pass through quarter waveplate 856 positioned in the path of coherent light beams 832 prior toilluminating holographic storage disk 238. Quarter wave plate 856 isconfigured to rotate the polarization orientation of coherent lightbeams 832 by approximately 45 degrees.

After leaving quarter wave plate 856, coherent light beams 832 areincident on holographic storage disk 238. The diffraction caused bydiffractive grating 844 causes each beam 832 to illuminate one of theencoded tracks 510, 520, 530 on holographic storage disk 238.

In the embodiment shown in FIG. 8, laser 840 and holographic storagedisk 238 are configured such that a substantial portion of light beams832 will be reflected from encoded tracks 590. The light beams 870reflected from the encoded tracks 590 of holographic storage disk 238are collectively referred to as reflected light beams 870. Reflectedlight beams 870 are reflected back through quarter wave plate 856 to PBS846. As a result of traveling through quarter wave plate a second time,reflected light beams 870 each have a polarization orientation that isrotated 90 degrees from coherent light beam 830.

As is well known in the art, a PBS will substantially transmit lightbeams of a particular polarization orientation and will substantiallyreflect light beams of an orthogonal polarization orientation. BecausePBS 846 is configured to substantially pass through coherent light beams832 leaving grating 844, PBS 846 substantially reflects beams 870towards detector module 380A. The light beams reflected from PBS 846toward detector module 380A are referred to herein as light beams 878.

In the illustrated embodiment, detector module 380A comprises a lens 858and a detector 848. Detector 848 comprises a multiple element detector.In certain embodiments, detector 848 is a six-element detector asdescribed in detail below with reference to FIG. 10. Lens 858 ispositioned in the path of reflected coherent light beams 878 between PBS846 and detector 848. Lens 858 is preferably a cylindrical lensconfigured to direct reflected coherent light beams 834 so that each ofthe reflected coherent light beams is incident on a different singlepair of detector elements in detector 848.

As would be appreciated by one of ordinary skill in the art, trackillumination module 744A may operate without lens 858. Lens 858 isuseful in embodiments in which grating 844 produces a smaller angularseparation between coherent light beams 832. Grating 844 is configuredto produce such a smaller angular separation in embodiments in which itis desirable to route light beams 832 and 870 in relative closeproximity to one another.

Light beams 878 leaving lens 858 are incident on detector 848 and eachpair of elements of the detector observes a diffraction pattern.Detector 848 provides the results of the diffraction patterns toabsolute position determinator 370. As noted, details of detector 848and absolute position determinator 370 are described below withreference to FIGS. 10 and 11.

Throughout this application, the term polarizing beam splitter (PBS)refers to any device configured to direct an incident light beam in adirection based on the polarization of the incident light beam. Forexample, embodiments of a PBS used in accordance with present inventionmay be a polarizing beam splitter cube, a thin film polarizer, a platepolarizer, a prism made of certain materials such as calcite, acustomized prism, and other devices now or later developed.

It should also be understood that light steering subsystem 360 may beconfigured to direct light beams 832 toward holographic storage disk 238at a desired angle. In such an embodiment, reflected light beams 870would be directed directly toward detector module 380A. As a result,light steering subsystem 360A would not require PBS 846 and quarter waveplate 856. It should also be appreciated by one of ordinary skill in theart, beam splitters may be used in place of grating 844 to form one ormore light beams 832.

FIG. 9 is an enlarged functional block diagram of the components of oneembodiment of a position tracking module 344B introduced above withreference to FIG. 3B. As noted above, position tracking module 350comprises a track illumination module 344B and an absolute positiondetector 370. In accordance with the embodiment illustrated in FIG. 9,track illumination module 344B comprises a laser 840, a light steeringsubsystem 360B, and a detector module 380B.

Laser 840 generates a coherent light beam 830 directed towardsholographic storage disk 238. Light beam 830 enters light steeringsubsystem 360B positioned between laser 840 and holographic storage disk238. Light steering subsystem 360B comprises a lens 842 and a grating844 sequentially aligned between laser 840 and holographic storage disk238.

In some embodiments, lens 842 is positioned between laser 840 andgrating 844 in the path of coherent light beam 830. Lens 842 ispositioned such that portions of coherent light beam 830 incident onholographic storage disk 238 will be focused coherent beam spots.

It would be appreciated by one of ordinary skill in the art that lens842 could be positioned in the path of coherent light beam 830 aftergrating 844. Similarly, it would be appreciated by one of ordinary skillin the art that track illumination module 344B may also operate withoutlens 842 positioned in light steering subsystem 360B. In saidembodiment, the portions of coherent light beam 830 incident onholographic storage disk 238 would not be focused beam spots.

Coherent light beam 830 leaving lens 842 then impinges on grating 844.Grating 844 is a diffractive grating that diffracts coherent light beam830 into one or more coherent light beams 832 directed towardsholographic storage disk 238. In the illustrated embodiment, grating 844diffracts or divides coherent light beam 830 into three coherent lightbeams 832. Grating 844 also allocates the power of light beam 830between coherent light beams 832, as is well-known in the art.

After leaving grating 844, coherent light beams 832 are incident onholographic storage disk 238. The diffraction caused by diffractivegrating 844 causes each beam to illuminate one of the encoded tracks onholographic storage disk 238 described above with reference to FIG. 5.

In the embodiment shown in FIG. 9, laser 840 and holographic storagedisk 238 are configured such that a substantial portion of light beams832 will be transmitted by holographic storage disk 238 through encodedtracks 590. The light beams transmitted through the encoded tracks 590of holographic storage disk 238 are collectively referred to astransmitted light beams 836. Transmitted light beams 836 are incident ondetector module 380B.

In the illustrated embodiment, detector module 380B comprises a lens 958and a detector 948. Lens 958 is positioned in the path of transmittedcoherent light beams 836 between holographic storage disk 238 anddetector 948. Detector 948 comprises a multiple element detector. Inembodiments of the present invention, detector 948 is six-elementdetector as described in detail below with reference to FIGS. 10. Lens958 is configured to direct transmitted coherent light beams 836 so thateach of the transmitted coherent light beams is incident on a differentsingle pair of detector elements in detector 948. In the illustrativeembodiments, lens 958 is a concave lens, although other lens may beimplemented in alternative embodiments.

As would be appreciated by one of ordinary skill in the art, trackillumination module 344B could operate without lens 858. Lens 858 isuseful, for example, in embodiments in which grating 844 produces asmall angular separation between coherent light beams 832. Grating 844is configured to produce such a small angular separation in embodimentsin which it is desirable to route light beams 832 and 836 in relativeclose proximity to one another.

Light beams 836 leaving lens 958 are incident on detector 948 and eachpair of elements of the detector observes a diffraction pattern.Detector 948 relays the results of the diffraction patterns to absoluteposition determinator 370. As noted, details of detector 948 andabsolute position determinator 370 are described below with reference toFIGS. 10 and 11.

FIG. 10 is an enlarged block diagram illustrating a detector used inembodiments of the present invention, referred to herein as detector1002. Detector 1002 is a six-element detector comprising a 3×2 array ofdetector elements 1004. The light beam reflected/transmitted by oneencoded track 590 (FIG. 5) on holographic storage disk 238 is incidenton a single pair of parallel detector elements in detector 1002.

When each of the encoded data tracks 590 are illuminated with a lightbeam, each track 590 causes a diffraction pattern at a different pair ofparallel detector elements 1004, and provides a signal to absoluteposition determinator 370 based on the diffraction pattern, as describedbelow with reference to FIG. 11.

It would be appreciated by one of ordinary skill in the art thatdetector 1002 may comprise more or less detector elements 1004. Detector1002 may use a single detector element 1004 to detect the lightreflected from each track 510, 520, 530. For example, detector elements1004 may be position-sensitive detector element such as a lateral-effectphotodiode that detects light reflected from a single track 510, 520,530. It should be appreciated that other types of detector elements maybe utilized in such embodiments depending on cost and desired accuracy.Similarly, detector 1002 may use more detector elements 1004 to detect adiffraction signal, such as in a CCD detector array.

FIG. 11 is diagram illustrating the diffraction pattern observed by anexemplary pair (1004A, 1004B) of detector elements 1004 illustrated inFIG. 10 in accordance with embodiments of the present invention.

The light from one of the three encoded tracks 510, 520, 530 isreconstructed using a push-pull detection method. Push-pull detection inaccordance with embodiments of the present invention occurs when afocused coherent light beam is reflected by/transmitted through track ona medium. The reflected/transmitted light is then detected in the farfield. The tracks cause the incident beam to be diffracted intodifferent orders. A split detector positioned at the far field detectsthe interference pattern between the directly reflected beam (0^(th)order) and the components diffracted by the tracks (+/−1 orders).

More specifically, as described above, in accordance with embodiments ofthe present invention, a holographic storage disk 238 having etchedtracks 590 thereon (FIG. 5) is illuminated with one or more light beams832 (FIGS. 8 and 9). Etched tracks 590 cause various portions of thereflected/transmitted light beams to experience different optical pathlengths prior to impinging on detectors 848 (FIG. 8) and 948 (FIG. 9).

In the case of the reflection embodiment illustrated in FIG. 8, thedifferent optical path lengths result from the fact that the portions ofa coherent light beam incident on a groove must travel through a longeroptical path than the portions that are incident on a mesa. However, inthe case of the transmission embodiment illustrate in FIG. 9, thedifferent optical path lengths results from the fact that portions of acoherent light beam incident on a mesa must travel farther than theportions incident on a groove. These different optical path lengthstraveled by the portions of coherent light beams reflected/transmittedby a groove than the light reflected/transmitted by a mesa causes thediffraction in the far field.

Moreover, as a focused coherent light beam spot scans across a track,the shifting of a groove relative to the focused spot causes a linearphase rotation in the far Fourier plane of the diffracted components.The phase of the undiffracted component (the 0^(th) order) isunaffected. The shift induced phase rotation causes the interferencebetween the diffracted and undiffracted components to becomeincreasingly constructive on one side of the photodetector, whereas itbecomes increasingly destructive on the opposite side. Thus, thedetector positioned in the far field detects an intensity differentialthat is proportional to the position of the pit relative to the focusedspot.

This intensity differential observed at each pair of detector elementsin detectors 848, 948 is converted to a signal and relayed to absoluteposition determinator 370 via data lines 124. The diffraction observedfrom each of the tracks results in a different piece of informationbeing relayed to absolute position determinator 370. Preferably, theintensity differential is normalized by the total intensity so that thepeaks are independent of the beam intensity.

Referring to the reconstruction of the illumination from quadraturetracks, the diffraction from tracks 510 and 530 results in thereconstruction of a sine and cosine curve respectively at absoluteposition determinator 370. The reconstructed sinusoids have peaks andvalleys at the edges of the grooves and zero crossings at the middle ofthe grooves and mesas. Zero crossings are areas where there are signalsthat change linearly with position. Furthermore, as noted, cosine track530 is configured such that the reconstructed cosine curve will be 90degrees out of phase from the reconstructed sine curve. This phase shiftcauses one curve or the other curve to always be in the linear range.

Referring next to the reconstruction of the illumination from addresstrack 520, the light diffracted from track 520 communicates to absoluteposition determinator 370 whether or not the focused lightbeam-illuminating track 520 has crossed an edge, either groove to mesaor mesa to groove. Absolute position determinator 370 keeps track ofthese changes in order to build up a record of a whole address code wordhaving n bits. As noted above, in the embodiments shown n is equal to12, and thus absolute position determinator 370 can determine theabsolute position of holographic storage disk 238 based on thereconstruction of any 12 bits in sequence. This is referred to as aframe invariant encoding scheme.

Absolute position determinator 370 is configured to read the signal fromdetectors 848, 948 only at particular times, i.e., times when absoluteposition determinator 370 expects an edge crossing. This is accomplishedby using the illumination from cosine track 530. Due to the fact thatcosine track 530 has a zero crossing right where address track 520 has agroove edge, absolute position determinator 370 uses the cosine zerocrossing to clock the state of the address track edges (+edge, −edge, orno edge, determined by a threshold) into a shift register. Once a codeword's worth is stored in the shift register, the edges are turned into1s and 0s (ie. integrate to get grooves and mesas) to get an LFSR codeword.

In another embodiment, absolute position determinator 370 is configuredto read the signal from detectors 848, 948 based on an oscillator thatis synchronized to the signal itself by means such as aphase-locked-loop (PLL). Such a method would require that holographicstorage disk 238 be moving at a relatively constant speed, but wouldallow aspects of the invention to be practiced without theimplementation of cosine track 530. Capabilities and techniques forimplementing PLLs are well-established in the art.

In embodiments of the present invention, this code word is used as anindex into a table to get the actual absolute position of holographicstorage disk 238.

In other embodiments, absolute position determinator 370 is configuredto clock an LFSR until it retrieves a code word matching the code wordreconstructed from the illumination of holographic storage medium. Inthis embodiment, absolute position determinator 370 counts the number ofclocks required to retrieve the matching code word, and the number ofclocks in turn provide the position of holographic storage disk 238.

In further embodiments of the present invention, absolute positiondeterminator 370 is configured to use the reconstructed sine and cosinecurves to provide fine tuning of the absolute position of holographicstorage disk 238. As noted, one or the other of the reconstructedsinusoids is always in the linear range. As a result, absolute positiondeterminator 370 is configured to interpolate within the availablelinear range to further determine the absolute position of holographicstorage disk 238.

It would be appreciated that methods other than push-pull detection maybe used in embodiments of the present invention. For example, detectors848, 948 could be configured to track the resulting light and darkpatterns as the focused light beams cross grooves. In other embodiments,detectors 848, 948 are configured to track the resulting light and darkpatterns as the focused light beams cross light and dark strips embossedon holographic storage disk 238.

It will be apparent to one or ordinary skill in the art that the methodof encoding address track 520 constitutes “return-to-zero” (RZ) encodingwherein the track pattern includes a mesa (zero level) in half of eachbit period regardless of whether a groove (one level) is present. Itwill also be readily apparent that one may use other methods ofencoding, including but not limited to “non-return-to-zero” encoding(NRZ) (the entire bit period is a groove or a mesa depending on whetherit's a one or a zero); “non-return-to-zero-inverted” encoding (NRZI)(the bit period contains an edge if the bit is a one, or no edge if thebit is zero), or “Manchester” encoding (the center of the bit periodcontains a groove-to-mesa edge for a zero, or a mesa-to-groove edge fora one).

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

Further features of alternative embodiments of the present invention aredescribed in US Patent Application No. 20040027668 entitled MediumPosition Sensing, filed on Feb. 14, 2006, which is hereby incorporatedby reference herein.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. For example, in the aboveembodiments the coherent light source 102 may be “always-on” and have nosignal line provided by the drive electronics to control its on/offstatus. As another example, optical steering subsystem 104 may not beimplemented as described above. In alternative embodiments, for example,optical steering subsystem 104 is configured to provide a fixed opticalbeam path; that is, the “steering” function does no involve any activedirection changing. Such embodiments are considered to be included inthe definition of the term “optical steering” or “light steering” asused herein. As another example, embodiments of the present inventionhave been described herein in the context of a holographic memory systemin which two light beams, a data beam and a reference beam, are utilizedto write data to the holographic storage medium. It should beappreciated, however, that embodiments of the present invention may beimplemented in any holographic memory system now or later developed. Onesuch contemplated application is a holographic memory system in which asingle light beam is utilized to provide “collinear holographicstorage.” Furthermore, in the. above embodiments, the object is aholographic storage medium in a holographic memory system. It should beappreciated, however, that the object may be any object on whichembossed tracks may be disposed. The present embodiments are, therefore,to be considered in all respects as illustrative and not restrictive.

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

1. A device for sensing an absolute position of a holographic storagemedium, comprising: a position tracking module comprising: a trackillumination module configured to illuminate the holographic storagemedium with one more light beams, and to detect one or more light beamsresulting from said illumination of said holographic storage medium; andan absolute position determinator configured to determine the absoluteposition of the holographic storage medium based on said one or morelight beams resulting from said illumination of said holographic storagemedium.
 2. The device of claim 1, wherein said track illumination modulecomprises: a source of light configured to generate a light beam; alight steering subsystem configured to divide said light beam into saidone or more light beams incident on said encoded object; and a detectormodule configured to detect said one or more light beams resulting fromsaid illumination of said encoded object by said incident light beams.3. The device of claim 2, wherein said holographic storage mediumcomprises: an object having one or more encoded tracks embossed therein.4. The device of claim 3, wherein said one or more encoded trackscomprise: one or more quadrature tracks; and an address track.
 5. Thedevice of claim 4, wherein said address track comprises: a frameinvariant encoded track.
 6. The device of claim 5, wherein said frameinvariant encoded track comprises: a code generated by a linear feedbackshift register (LFSR).
 7. The device of claim 4, wherein said quadraturetracks comprise: a sine track configured such that the reconstruction ofillumination from said sine track results in the reconstruction of asine curve; and a cosine track shifted from said sine track such thatthe reconstruction of illumination from said cosine track results in thereconstruction of a cosine curve phase shifted 90 degrees from said sinecurve.
 8. The device of claim 7, wherein said detector module isconfigured to detect a light beam reflected from each of said encodedtracks.
 9. The device of claim 8, wherein said light steering subsystemcomprises: a grating configured to redirect said light beam into threeillumination light beams; a polarizing beam-splitter (PBS) configured todirect said three illumination light beams towards said encoded tracks;and a quarter wave plate configured to rotate the polarization of saidthree illumination light beams; wherein said three light beams reflectedfrom said encoded tracks are directed back through said quarter waveplate and PBS towards said detector module.
 10. The device of claim 9,wherein said detector module comprises: a six-element split detectorconfigured to detect diffraction patterns resulting from the reflectionof said illumination light beams from said encoded tracks; wherein saiddiffraction pattern for each said reflected light beam is detected at asingle pair of split element detectors; and wherein said detector moduleis configured to provide said absolute position determinator withsignals based on said diffraction patterns.
 11. The device of claim 7,wherein said detector module is configured to detect a light beamtransmitted through each of said encoded tracks.
 12. The device of claim11, wherein said light steering subsystem comprises: a gratingconfigured to redirect said light beam into said three light beamsconfigured to illuminate said encoded tracks.
 13. The device of claim12, wherein said detector module comprises: a six-element split detectorconfigured to detect diffraction patterns resulting from thetransmission of said illumination light beams from said encoded tracks;wherein said diffraction pattern for each said transmitted light beam isdetected at a single pair of split element detectors; and wherein saiddetector module is configured to provide said absolute positiondeterminator with signals based on said diffraction patterns.
 14. Thedevice of claim 1, wherein said holographic storage medium containsholographically stored information.
 15. A holographic memory system,comprising: an encoded holographic storage medium configured toholographically store information; at least one source of coherentlight; and a position tracking module comprising: a track illuminationmodule configured to illuminate said encoded holographic storage mediumwith one more light beams, wherein said track illumination module isconfigured to detect one or more light beams resulting from saidillumination of said encoded holographic storage medium; and an absoluteposition determinator configured to determine the absolute position ofsaid encoded holographic storage medium based on said one or more lightbeams resulting from said illumination of said encoded holographicstorage medium.
 16. The system of claim 15, wherein said trackillumination module comprises: a source of light configured to generatea light beam; a light steering subsystem positioned between said sourceof light and said encoded holographic storage medium configured toredirect said light beam into said one or more light beams incident onsaid encoded holographic storage medium; and a detector moduleconfigured to detect one or more light beams resulting from saidillumination of said encoded holographic storage medium by said incidentlight beams.
 17. The system of claim 16, wherein said encodedholographic storage medium comprises: a medium having one or moreencoded tracks embossed therein.
 18. The system of claim 17, whereinsaid one or more encoded tracks comprise: three encoded trackscomprising: a plurality of quadrature tracks; and an address track. 19.The system of claim 18, wherein said address track comprises: a frameinvariant encoded track.
 20. The system of claim 19, wherein said frameinvariant encoded track comprises: a code generated by a linear feedbackshift register (LFSR).
 21. The system of claim 20, wherein said detectormodule is configured to a light beam transmitted through each of saidencoded tracks.
 22. The system of claim 21, wherein said light steeringsubsystem comprises: a grating configured to redirect said light beaminto said three light beams configured to illuminate said encodedtracks.
 23. The system of claim 22, wherein said detector modulecomprises: a six-element split detector configured to detect diffractionpatterns resulting from the transmission of said illumination lightbeams from said encoded tracks; wherein said diffraction pattern foreach said transmitted light beam is detected at a single pair of splitelement detectors; and wherein said detector module is configured toprovide said absolute position determinator with signals based on saiddiffraction patterns.
 24. The system of claim 18, wherein saidquadrature tracks comprise: a sine track configured such that thereconstruction of illumination from said sine track results in thereconstruction of a sine curve; and a cosine track shifted from saidsine track such that the reconstruction of illumination from said cosinetrack results in the reconstruction of a cosine curve phase shifted 90degrees from said sine curve.
 25. The system of claim 24, wherein saiddetector module is configured to detect a light beam reflected from eachof said encoded tracks.
 26. The system of claim 25, wherein said lightsteering subsystem comprises: a grating configured to redirect saidlight beam into three illumination light beams; a polarizingbeam-splitter (PBS) configured to direct said three illumination lightbeams towards said encoded tracks; and a quarter wave plate configuredto rotate the polarization of said three illumination light beams;wherein said three light beams reflected from said encoded tracks aredirected back through said quarter wave plate and PBS towards saiddetector module.
 27. The system of claim 26, wherein said detectormodule comprises: a six-element split detector configured to detectdiffraction patterns resulting from the reflection of said illuminationlight beams from said encoded tracks; wherein said diffraction patternfor each said reflected light beam is detected at a single pair of splitelement detectors; and wherein said detector module is configured toprovide said absolute position determinator with signals based on saiddiffraction patterns.
 28. The holographic memory system of claim 26,wherein each of the three illumination light beams is incident on one ofthe quadrature or address track of the holographic storage medium, andwherein none of the three illumination light beams is incident on thesame quadrature or address track of the holographic storage medium. 29.The holographic memory system of claim 15, wherein said absoluteposition determinator determines the absolute position of saidholographic storage medium in relation to the optical components of saidholographic memory system.
 30. The holographic memory system of claim29, wherein the holographic storage medium translates or rotates withinsaid holographic memory system.
 31. A method of determining the absoluteposition of an encoded holographic storage medium comprising: (a)illuminating said encoded holographic storage medium with one or morelight beams; (b) detecting the resulting illumination from said encodedholographic storage medium; and (c) determining the absolute position ofsaid encoded holographic storage medium based on the detectedillumination from said encoded medium.
 32. The method of claim 31,wherein said resulting illumination from said encoded holographicstorage medium is reflected from said encoded holographic storage mediumand is used for determining the absolute position of said encodedholographic storage medium.
 33. The method of claim 31, wherein saidresulting illumination from said encoded holographic storage medium istransmitted by said encoded holographic storage medium and is used fordetermining the absolute position of said encoded medium.
 34. The methodof claim 31, wherein steps (a), (b), and (c) are performed in connectionwith reading, writing, pre-curing, post-curing, or write verifying saidholographic storage medium.